Spatial diversity wireless communications (radio) receiver

ABSTRACT

A combiner for use in a spatial diversity radio receiver which receives multiple data signals through spaced apart antennae. The combiner includes a receiving component configured for receiving strength-indicative signals, each strength-indicative signal being indicative of the strength of one of the received data signals, and demodulated data signals. A control signal generating component configured for generating control signals generates control signals responsive to the strength-indicative signals. A combining component configured for combining signals combines, in linear proportions determined by the control signals, those of the demodulated data signals which are both above a predetermined strength threshold level (“combiner threshold”) and differ in strength by less than a predetermined margin of preferably between 3 dB and 12 dB (e.g. 6 dB) to provide a combined output data signal. When more than the margin separates the signal strengths, only the strongest signal is used. A spatial diversity wireless communications (radio) receiver includes multiple receiving components having spaced-apart antennae, each receiving component providing both a signal indicative of received signal strength and a demodulated signal output, a combiner as aforesaid to provide an output data signal and means for evaluating the output data signal. Preferably, rapidly-fading signals are identified by evaluating second derivative signals of the strength-indicative signals, and weighted accordingly if a non-fading signal is available.

FIELD OF THE INVENTION

This invention relates to spatial diversity wireless communications(radio) receivers, particularly for data signals.

BACKGROUND OF THE INVENTION

Land mobile radio systems as used for dispatch applications, and alsomany wireless cellular systems, use frequency modulation which hasproven to be well suited to the application by reason of immunity toimpulse noise which is common in the vehicular environment. Such systemsare increasingly being required to transmit data as well as analogspeech and advances in computer technology have increased the demand forhigher bit rates for data transfer.

Radio frequencies are regulated. While demands for higher bit rates arewide spread, regulatory agencies have not increased the bandwidth neededto facilitate high speed data transmission. In fact, the trend is in theopposite direction. In 1997 the Federal Communications Commissionmandated the use of channels which are one half or one quarter as wideas those previously authorized. As taught by Shannon and Nyquist, thereis a proven relationship between the bit rate of a channel, thebandwidth of the channel and the signal to noise ratio required todecode the data accurately. As the bit rate increases, all else beingequal, the signal to noise ratio required to decode the data is alsoincreased, and thus the range of the radio system is reduced as the bitrate increases.

Land mobile and cellular channels differ from those used in fixedmicrowave point to point services and satellite systems by virtue ofreflections and fading of the signals. Signals arriving at or from awireless communications device such as a mobile radio receiver orcellular telephone are almost always comprised of a complex amalgam ofwaves, some directly from the sending antenna and others reflected fromstationary and moving objects. In the worst case scenario, the totalreceived signal is composed of reflected signals. The resulting waveformcaused by the combination of reflected signals (worse case) and/ordirect signal plus reflected signals, is subject to cancellation orreinforcement in the amplitude domain as well as distortion in the timedomain resulting from propagation delays over the varying length pathstaken by reflected signals. Both the amplitude and time distortions makedecoding of the signals more difficult. It is not uncommon forcancellation to reduce the incoming signal to a level far below thethreshold required for reliable decoding by the receiver. This effect isreferred to as multi-path fading.

In data systems, such cancellations or “drop outs” erase portions of thedesired bit stream. The duration of the erasure is a function of theaverage signal strength, the wavelength of the radio signal, the speedof the vehicle (where the wireless device is being operated in avehicle) and that of moving reflectors in the vicinity. Forward ErrorCorrection (FEC) is a common technique for solving this erasure problem.Redundant information is added to the transmitted data to allow for apredicted level of erasures and recovery of the original data withoutretransmission. FEC is useful but as the bit rate increases, more andmore redundancy must be added which leads to diminishing returns. Theredundancy reduces the effective bit rate of the system.

Another solution to problems caused by multi-path fading is to increasethe complexity of the receiving system. Fading can be mitigated by usingmultiple receivers and multiple antennas. Such systems are often calleddiversity receivers since they are based on spatial diversity. Two ormore receivers with separate antennae spaced an appropriate distanceapart from each other so that the received signals are non-correlatedgive rise to probabilities that destructive interference experienced atone antenna may not be present on another.

Spatial diversity receiving systems generally use one of three differentclasses of techniques to combine the multiple signals, being: (i)selection combining whereby the best signal is chosen based onassessment of signal strength (i.e. the signal having the bestsignal-to-noise ratio); (ii) equal gain combining whereby all signalsare combined together regardless of the strength of any individualsignal; and (iii) optimal combining whereby the signals are combinedproportionally based on their individual strengths. Only the latterattempts to make use of the maximum possible information contentavailable from all signals to yield optimal performance. However, inpractice, it has been difficult to design combiner circuitry whicheffectively combines signals on such an optimal basis, the problem beingto develop effective and practical algorithms for determining theweights to be applied. Many known optimal combiners use complexequalizers to implement an estimation of the received symbol sequenceswhich is then used to proportionally weight the received signals forcombining purposes. Such systems are exemplified by the following patentreferences.

Each of U.S. Pat. No. 5,499,272 (Bottomley) and U.S. Pat. No. 5,701,333(Okanoue et al) apply complex estimation algorithms to, in effect,produce a synthesized received data stream. U.S. Pat. No. 5,862,192(Huszar et al) also applies an estimation algorithm but it comparesestimated sequences to sample sequences, and selects received sequenceson the basis of this comparison. U.S. Pat. No. 5,901,174 (Richard)applies weightings to the received channels which are derived fromchannel error estimations based on a global estimation algorithm.Another system, described in U.S. Pat. No. 5,640,695 (Fitzgerald), usesa continuously switching logic control mechanism for audio signals (thisbeing a type of selective combiner). U.S. Pat. No. 4,972,434 (Le Polozecet al) uses an adaptive (feedback type) equalizer to derive a distortionfactor which is used to weight signal strength measurements for acombiner such that the distortion factor is based on the combined signalproduced by the combiner.

Accordingly, there is a need for an effective means of optimallycombining signals in a spatial diversity receiver which is less complexthan those of the prior art.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a combiner for use ina spatial diversity radio receiver which receives multiple data signalsthrough spaced apart antennae. The combiner includes means for receivingstrength-indicative signals, each strength-indicative signal beingindicative of the strength of one of the received data signals, anddemodulated data signals. The combiner provides means for generatingcontrol signals responsive to the strength-indicative signals and forcombining, in linear proportions determined by the control signals,those demodulated data signals which are both above a predeterminedcombiner strength threshold level and differ in strength by less than apredetermined margin of preferably between 3 dB and 12 dB, to provide acombined output data signal. The demodulated data signals are therebycombined in proportion to an amount by which they differ relative to thepredetermined margin (e.g. 6 dB), and the greatest proportion is of thestrongest of the data signals. A digital signal processor preferablyprovides the generating and combining means. The generating meanscomprises means for evaluating the strength-indicative signals and, toaddress the situation of rapid signal fading, the evaluating means mayalso include means for producing a second derivative signal for eachstrength-indicative signal whereby the control signal is generatedaccording to a predetermined combination of the strength-indicativesignals and second derivative signals. Preferably, the combiner includesadaptive DC bias compensation means to adjust the relative DC levels ofthe received demodulation data signals, wherein the compensation isalways done but the DC level used to do so is only adjusted when bothdemodulated data signal strengths are above another predeterminedthreshold level which is referred to herein as the DC bias compensationthreshold.

In accordance with a further aspect of the invention there is provided aspatial diversity radio receiver comprising multiple receivingcomponents for receiving data signals through antennae each of which isassociated with one of the receiving components and spaced apart fromthe other antenna(e) a predetermined distance. Each receiving componentcomprises circuitry for providing a signal indicative of the strength ofthe received data signal and a demodulated data signal, a combineraccording to the foregoing and circuitry for evaluating the combinedoutput data signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made to the accompanying drawings which show, byway of example, a presently preferred embodiment of the presentinvention, and in which:

FIG. 1 is a schematic diagram of a first receiving component of awireless receiver;

FIG. 2 is a schematic diagram of a second receiving component of awireless receiver having an antenna spaced apart from that of the firstreceiving component to result in non-correlation of the signals receivedthereby;

FIG. 3 is a schematic diagram of a digital signal processor implementingcircuitry for processing signals received from the first and secondreceiving components of the wireless receiver; and,

FIG. 4 is a graph illustrating the combination of received signals.

DESCRIPTION OF THE ILLUSTRATED PREFERRED EMBODIMENT

Referring to FIGS. 1 and 2, the receiving components shown therein maybe identical, and the same reference numerals are used to refer to thesame parts. Only the signals generated by the two receiving componentsare differentiated herein by being designated with the suffixes 1 and 2respectively. While a wireless communications (radio) receiver utilisingtwo receiving components with spaced apart antennas is described herein,it will be understood by persons skilled in the art that similarprinciples to those described below may be applied to implement theinvention in receivers employing three or more receiving components withassociated spaced apart antennas. The terms “radio” and “wirelesscommunications” are intended to have the same meaning, and are usedinterchangeably, herein; they are not intended to be limited to anyparticular frequency range or modulation scheme.

The receiving components each comprise an antenna 2 and FM receiver 4which in turn comprises a radio frequency (RF) section 6, anintermediate frequency (IF) section 8, and a baseband section 10 whichdetects and regenerates a baseband signal (RXA1 and RXA2). The IFsections 8 provide analog radio signal strength indicator (RSSI) signalsRSSIA1 and RSSIA2 respectively, conveniently derived by summing stagecurrents in limiting amplifiers in each IF section, although other knowntechniques for deriving such signals may be utilised. In this manner,the signal strength is measured on the basis of the power of the signal.However, for another embodiment it might, alternatively, be chosen touse circuitry which measures the signal strength on the basis of thesignal amplitude and, in such case, the subsequent circuitry would berequired to square the measured value. For the exemplary embodiment, theIF sections are designed so that the RSSI signals exhibit a temperaturestable monotonic logarithmic characteristic over a range of greater than70 dB.

For the illustrated embodiment, the baseband section incorporates an FMdetector circuit, in this case being a quadrature detector which splitsthe IF signal into two parts of which one part passes through a networkwith a phase shift of 90 degrees plus a shift proportional to the IFdeviation from center frequency, while the other part passes straightthrough; multiplies the shifted and unshifted parts together; andselects the baseband frequency portion of the multiplier output spectrumto provide baseband analog signals RXA1 and RXA2.

The signals RSSIA1, and RSSIA2, RXA1 and RXA2 are passed throughrespective 3-pole and 1-pole anti-aliasing filters 12 and 14 which limittheir bandwidth to exclude noise and components at frequencies of morethan half the sampling rate applied in analog to digital converters(ADC) 16 and 18, to which are passed the signals output from the filters12 and 14. Typically, the ADC 16 used to digitize the RSSIA1 and RSSIA2signals to produce signals RSSID1 and RSSID2, respectively, is a sampleand hold converter which tracks an analog input signal during a samplemode and holds it fixed during a hold mode to the instantaneous value ofthe signal at each transition from the sample to the hold mode.Typically, the ADC 18 used to digitize the RXA1 and RXA2 signals toproduce signals RXD1 and RXD2, respectively, is an oversamplingconverter comprising an input signal conditioning circuit, adifferential fifth order delta-sigma modulator, a 64× oversamplingdecimation circuit from which output signals pass to a serial interface25 (see FIG. 3). A typical sampling rate for both converters 16 and 18is at least 2.5 times the bit rate of the data being received, and inany case, is greater than the Nyquist rate.

Referring now to FIG. 3, a digital signal processor (DSP) 20 is used toimplement, through suitable programming and techniques well known tothose skilled in the art of DSP selection and programming, a combiner inaccordance with the invention having the components shown in FIG. 3.With reference to this illustration, however, it is to be noted that theillustrated embodiment is directed to an FM modulation scheme using alimiter-discriminator-type demodulator (e.g. for a frequency in the450-800 MHZ range) and a hardware implementation of the demodulator isused. Alternatively, for some embodiments it could instead be chosen toimplement portions of the demodulator by a DSP. Also, for alternativeembodiments other modulation schemes may be selected at the same ordifferent frequency ranges and the receivers for such embodiments maymake more extensive use of a DSP than the embodiment shown in FIG. 3.All references herein to a component refer to circuitry and thecircuitry described herein may, for some embodiments, be implemented andprovided by a DSP.

For the DSP processing shown by FIG. 3, the digitized inputs RXD1 andRXD2 are applied to an adaptive DC bias compensation filter 22 whichcompares their DC levels and applies a compensation signal to one of thesignals through an adder 24 so as to adjust the relative DC levels ofthe received demodulated data signals. This compensation is done in allcases and, in addition, in cases where both of the data signals areabove a predetermined DC bias compensation threshold such as, forexample, −80 dBm, the DC level of the compensation signal used to do sois calibrated (i.e. adjusted). This calibration is done by applying asingle pole digital filter to the difference of the digitized inputsRXD1 and RXD2. The purpose of the DC compensation circuitry is tocompensate for differences in frequency, modulation level and otherfactors which may influence the DC bias levels of demodulated signalsfrom the channels received.

The DC-processed RXD signals are then passed to multiplier circuitry, inwhich they are multiplied in a ratio α: 1−α a in multipliers 26, 28whereby the RSSI signals determine the control signals α and 1−α, withthe control signal a being produced by generating means provided by theDSP. The RSSI signals pass through evaluating means, provided by digitalfilters 30, 32 and converter circuits 34, 36, which adjust them tosignals proportional to the dBm levels of the original RSSI signals,these signals being shown in FIG. 3 as dBm1 and dBm2. The multipliersare only enabled when dBm1 and dBm2 are both: (i) above a predeterminedstrength threshold level referred to herein as a combiner threshold (forexample −123 dBm) below which reliable demodulation is not possible;and, (ii) differ by less than a predetermined margin, the marginpreferably being programmable between 3 and 12 dB and a margin of 6 dBbeing selected for the embodiment described herein. The RXD signals aresummed by combiner circuitry 38 to provide a combined RXD signalaccording to the following expression, provided that the dBm1 and dBm2signals differ by less than the margin (e.g. 6 dB):

-   -   RXD combined=a×RXD1+(1−a)×RXD2        -   where 0≦α≦1 and        -   α=0.5+(dBm1−dBm2)/(2×margin)        -   where margin=6 dB (for the illustrated embodiment described            herein).

The combiner is controlled to provide various different resultsaccording to the dBm1 and dBm2 values. If there is more than one RSSIsignal above the threshold at which the multipliers are operative, andthe strengths of these signals differ by less than 6 dB, a linearcombination of the RXD signals is effected according to the aboveexpression. This results in a simple averaging of the adjusted RXDsignals if the RSSI signals are equal.

A signal whose RSSI is more than 6 dB below that of the strongest signalis normally ignored. However, when an FM wireless modulation scheme anda demodulation hard-limiting circuit are used, a special type ofdegradation may occur. This is when rapid signal fading happens andresults in a high negative slope followed by a high positive slope onthe RSSI signal. In this case, even if the fading channel signal has ahigher power level than the other(s), the lower power signal(s) may beless degraded. To handle this phenomenon the second derivative of theRSSI signal is, firstly, calculated using a digital filter which isprovided as part of the converters 34,36. Secondly, the RSSI signal isadded with this second derivative multiplied by an appropriate scalingfactor. The resulting dBm1 and dBm2 signals are biased in such a waythat the combining factor α accounts for this effect when it iscalculated.

The decoding of the digital signals is done in many stages and, as willbe recognized by persons skilled in the art, those stages will differdepending on the modulation scheme to be selected for use. The combiningprocess of the present invention has to take place at a stage where theinformation symbols are similar enough that they may be combinedlinearly according to the combiner circuitry described herein. In theillustrated embodiment the combiner circuitry is operative at theinitial stage of decoding but for other modulation schemes, such as QAM(Quadrature Amplitude Modulation), this combining process must be donefurther down the parallel decode process of the invention, for example,after symbol alignment and amplitude normalization.

FIG. 4 illustrates the operation of the combiner. The lower two graphsplot the outputs of the converters 34,36 for two channels throughsuccessive samples, while the uppermost graph illustrates theproportions in which the RXD signals are combined by the combinercircuit 38 with 1 representing a combination entirely formed by theupper channel, and 0 representing a combination formed entirely by thelower channel. Thus, along the time axis of the graphs, the signal ofthe lower channel from about 100 to 1100 is more than 6 dB greater thanthat from the upper channel, so a equals 0 and the lower channel aloneis selected (i.e. in this case only the stronger signal is selected). Ataround 1700, both signals are of similar strength (α=0.5) and they arethus simply averaged. At about 2300, the signals differ by just lessthan 6 dB, with the upper channel greater, and a linearly apportionedoutput is taken, according to 0.9 from the upper channel and 0.1 fromthe lower channel. It should be noted that the combiner works on asample-by-sample basis (the samples being the digital output produced byblocks 16 and 18 at a rate Fs) and thus the operation of the combiner iscontrolled by the RSSI signals over the period of a sample.

To recover the original digital signal in accordance with normalrecovery processing methods, the output from the combiner circuit 38 ispassed to a digital filter 40, and thence to a polyphase interpolationfilter 42 whose purpose is to provide an accurate interpolation of anoriginal signal produced by a Nyquist sampling process. It uses anover-sampling process to interpolate additional points to a curvereconstructing the original signal. The oversampled signal reducestiming jitter, as compared to the Nyquist sampled signal, and enablesmore accurate recovery of symbol timing by a phase-locked loop 44implemented by the DSP, which loop provides a timing output to anintegrate and dump module 46 implemented by the DSP.

The output provided by the integrate and dump module 46 for each symbolis passed to a decision circuit 48 which evaluates the symbol, typicallybased upon a decision feedback method in which both the value of theoutput and the decisions made in respect of previous symbols are used toprovide a most likely estimate of the symbol being decoded.

It will be noted that, in operation, the digital signal processorcontinuously evaluates, in parallel, the signals received by eachreceiving component utilised, and combines data from these signals suchas to recover and combine information from each channel deemed able tocontribute to correct evaluation of the received data. The evaluationtests and combination techniques described above are those presentlybelieved to provide the best chance of correctly evaluating symbols of adata transmission, but these may be varied within the scope of theappended claims with a view either to improving performance, simplifyingimplementation, or taking advantage of improved DSP or other technologyused to implement the digital circuits of the receiver. Although it isconsidered that use of a suitably programmed DSP is presently theoptimum technology for implementing digital functions of the invention,other technology capable of implementing the same functions may, ofcourse, be used. The individual circuit functions and processingfunctions utilised in the receiver are, individually, well understood bythose skilled in the art, and although particular implementations ofthese functions may have been described, it should be understood thatfunctionally equivalent or superior implementations may be substituted.Likewise, particular functions may be performed at different points inthe receiver if functionally equivalent results are obtained.

1. A combiner for use in a spatial diversity radio receiver whichreceives multiple data signals through spaced apart antennae, saidcombiner comprising, for two received data signals: (a) means forreceiving two strength-indicative signals, each said strength-indicativesignal being indicative of a strength of one of said two received datasignals, and two demodulated data signals for each said two receiveddata signals; (b) means for generating control signals responsive tosaid strength-indicative signals; and, (c) means for combining saiddemodulated data signals in specific proportions determined by saidcontrol signals where said demodulated data signals are above apredetermined combiner threshold, to provide a combined output datasignal, said specific proportions being: (i) where a difference betweenstrengths of said demodulated data signals is more than a predeterminedmargin: 100% of only a strongest of said demodulated data signals; and,(ii) where said difference between strengths of said demodulated datasignals is less than said predetermined margin: for said demodulateddata signal which is a strongest of said demodulated data signals, 50%plus a second percentage corresponding to a ratio between one-half ofsaid difference between strengths and said predetermined margin; and,for said other demodulated data signal, 50% less said second percentage.2. A combiner according to claim 1, wherein said margin is between 3 dBand 12 dB.
 3. A combiner according to claim 2, wherein said margin is 6dB.
 4. A combiner according to claim 3, wherein said generating andcombining means are provided by a digital signal processor.
 5. Acombiner according to claim 4, wherein said generating means comprisesmeans for evaluating said strength-indicative signals.
 6. A combineraccording to claim 5, wherein said evaluating means comprises means forproducing a second derivative signal for each said strength-indicativesignal and said control signals are generated according to apredetermined combination of said strength-indicative signals and secondderivative signals.
 7. A combiner according to claim 5, and furthercomprising DC bias compensation means for adjusting the relative DClevels of the received demodulated data signals wherein saidcompensation means calibrates a level of a DC offset signal used forsaid adjusting when the strengths of said demodulated data signals areabove a predetermined DC bias compensation threshold.
 8. A combiner foruse in a spatial diversity radio receiver which receives multiple datasignals through spaced apart antennae, said combiner comprising, for tworeceived data signals: (a) a receiving component configured forreceiving strength-indicative signals, each of two saidstrength-indicative signal being indicative of a strength of one of saidtwo received data signals, and a demodulated data signal for each saidtwo received data signals; (b) a control signal generating componentconfigured for generating control signals responsive to saidstrength-indicative signals; and, (c) a combining component configuredfor combining said demodulated data signals in specific proportionsdetermined by said control signals where said demodulated data signalsare above a predetermined combiner threshold, to provide a combinedoutput data signal, said specific proportions being: (i) where adifference between strengths of said demodulated data signals is morethan a predetermined margin: 100% of only a strongest of saiddemodulated data signals; and, (ii) where said difference betweenstrengths of said demodulated data signals is less than saidpredetermined margin: for said demodulated data signal which is astrongest of said demodulated data signals, 50% plus a second percentagecorresponding to a ratio between one-half of said difference betweenstrengths and said predetermined margin; and, for said other demodulateddata signal, 50% less said second percentage.
 9. A spatial diversityradio receiver comprising: (a) multiple receiving components forreceiving data signals through antennae, each said antenna associatedwith one said receiving component and being spaced apart a predetermineddistance, each said receiving component comprising circuitry forproviding a signal indicative of the strength of said received datasignal and a demodulated data signal; (b) a combiner according to claim1; and, (c) circuitry for evaluating said combined output data signal.10. A spatial diversity radio receiver comprising: (a) multiplereceiving components for receiving data signals through antennae, eachsaid antenna associated with one said receiving component and beingspaced apart a predetermined distance, each said receiving componentcomprising circuitry for providing a signal indicative of the strengthof said received data signal and a demodulated data signal; (b) acombiner according to claim 2; and, (c) circuitry for evaluating saidcombined output data signal.
 11. A spatial diversity radio receivercomprising: (a) multiple receiving components for receiving data signalsthrough antennae, each said antenna associated with one said receivingcomponent and being spaced apart a predetermined distance, each saidreceiving component comprising circuitry for providing a signalindicative of the strength of said received data signal and ademodulated data signal; (b) a combiner according to claim 6, and, (c)circuitry for evaluating said combined output data signal.
 12. A spatialdiversity radio receiver comprising: (a) multiple receiving componentsfor receiving data signals through antennae, each said antennaassociated with one said receiving component and being spaced apart apredetermined distance, each said receiving component comprisingcircuitry for providing a signal indicative of the strength of saidreceived data signal and a demodulated data signal; (b) a combineraccording to claim 7; and, (c) circuitry for evaluating said combinedoutput data signal.
 13. A spatial diversity radio receiver comprising:(a) multiple receiving components for receiving data signals throughantennae, each said antenna associated with one said receiving componentand being spaced apart a predetermined distance, each said receivingcomponent comprising circuitry for providing a signal indicative of thestrength of said received data signal and a demodulated data signal; (b)a combiner according to claim 8; and, c) circuitry for evaluating saidcombined output data signal.